1. Field of the Invention
The present invention relates to a thermally-assisted magnetic recording method where a write magnetic field and heat are applied to a hard disk medium to write data thereon, and also relates to a hard disk drive and a hard disk medium to which this recording method is applicable.
2. Description of the Related Art
Recently, hard disk drives have been improved in recording density, and magnetic heads and hard disk media of improved performance have been demanded accordingly. Among the magnetic heads, a composite thin-film magnetic head has been used widely. The composite thin-film magnetic head has such a structure that a read head including a magnetoresistive element (hereinafter, also referred to as MR element) intended for reading and a write head including an induction-type electromagnetic transducer intended for writing are stacked on a substrate. In a hard disk drive, the magnetic head is mounted on a slider that flies slightly above the surface of the hard disk medium.
A hard disk medium has a magnetic recording layer made of an aggregate of many microscopic magnetic grains. Each magnetic grain has a single-domain structure. A single recording bit of the hard disk medium is composed of a plurality of magnetic grains that align at least in the track width direction. For improved recording density, it is necessary to reduce asperities at the borders between adjoining recording bits. To achieve this, the magnetic grains must be made smaller. However, making the magnetic grains smaller causes the problem that the thermal stability of magnetization of the magnetic grains decreases with decreasing volume of the magnetic grains. To solve this problem, it is effective to increase the anisotropic energy of the magnetic grains. However, increasing the anisotropic energy of the magnetic grains leads to an increase in coercivity of the magnetic recording layer, and this makes it difficult to perform data writing with existing magnetic heads.
To solve the aforementioned problems, there has been proposed a technology so-called thermally-assisted magnetic recording. The technology uses a hard disk medium having a magnetic recording layer of high coercivity. When writing data, a magnetic field and heat are simultaneously applied to the area of the magnetic recording layer where to write data, so that the area rises in temperature and drops in coercivity for data writing.
In conventional recording methods using only magnetism, contributive factors to an increase in linear recording density have been dominated mainly by a large gradient of change in write magnetic field in the track direction. In addition to this, for thermally-assisted magnetic recording, a change in temperature occurring in the magnetic recording layer in the track direction due to heat applied to the hard disk medium and a change in coercivity occurring in the magnetic recording layer in the track direction due to the aforementioned temperature change are also contributive factors to an increase in linear recording density. Thermally-assisted magnetic recording is therefore considered to be highly effective in increasing the linear recording density.
For thermally-assisted magnetic recording, the gradient of the change in coercivity of the magnetic recording layer in the track direction is expressed by the product of the gradient of the change in coercivity with respect to the change in temperature and the gradient of the change in temperature with respect to the change in position, as described in, for example, “Robert E. Rottmayer et al; IEEE Transactions on Magnetics, Vol. 42, No. 10, October, 2006, p. 2417-2421”. Therefore, it has conventionally been thought that a preferable property of the magnetic recording layer of a hard disk medium for use in thermally-assisted magnetic recording is a large gradient of the change in coercivity with respect to the change in temperature.
In practice, however, there is a problem that sufficient signal-to-noise ratios cannot be obtained at high linear recording densities even if a hard disk medium with a magnetic recording layer that exhibits a large gradient of the change in coercivity with respect to the change in temperature is used to perform thermally-assisted magnetic recording.